This project will develop a catheter and associated instrumentation for the continuous measurement of cardiac output using the thermodilution technique. Our system will heat the blood directly using either high frequency ultrasound, Nd:YAG lasers or microwave energy. Similar to other thermodilution methods, the heater will be located in the right atrium, and will apply energy to the blood to cause heating. A thermistor located at the end of the catheter in the pulmonary artery will measure the resulting temperature rise of the blood and use that measure to predict cardiac output. As this catheter will not use bolus injections of saline, it will avoid fluid overload of the blood system. It will measure cardiac output more accurately than thermodilution catheters that use resistive heaters to heat the blood because direct heating can couple more than six times the energy into the blood than a resistive heater. The final system and catheter will therefore be capable of measuring cardiac output up to 10 times per minute with greater than 95% accuracy. The measurement protocol will be completely automated, making routine application of this technique possible. This accuracy and measurement continuity will be achieved while reducing the risk of complications that can result from competing techniques, either fluid overload or spontaneous blood clotting resulting from overheating. PROPOSED COMMERCIAL APPLICATION: This project will develop a catheter and system that will be capable of measuring cardiac output accurately and continuously. It offers all cardiologists and critical care facilities measurements that are not currently achievable, and could dramatically increase the quality of care in these facilities.